The cryogenic separation of air is a well established industrial process. Cryogenic air separation involves the filtering of the feed air to remove particulate matter and compression of that clean air to supply the energy required for the separation. Following the air compression the feed air stream is cooled and cleaned of the high boiling contaminants, such as carbon dioxide and water vapor, and then separated into its components by cryogenic distillation. The separation columns are operated at cryogenic temperatures to allow the gas and liquid contacting necessary for separation by distillation and the separated products are then returned to ambient temperature conditions versus the cooling air stream. The separation columns are commonly used to produce oxygen, nitrogen, argon and the rare gases present in the feed air. The typical oxygen purity available from cryogenic air separation can range from enriched air to the high purity oxygen considered standard for the industry. Enriched air product which may range from 25% oxygen to perhaps 50% oxygen is often used in low grade combustion type applications, such as blast furnaces. Higher purity oxygen product such as 50-95% oxygen is often used for applications where the added oxygen content is beneficial but the remaining nitrogen is not a serious drawback. Typical applications can include some combustion purposes, chemical processes, and secondary waste-water treatment. The conventional high purity oxygen product which is nominally referred to as 99.5% oxygen is the usual product purity associated with cryogenic air separation. The conventional 99.5% oxygen associated with air separation industry is commonly used for a range of applications including metal cutting and working operations and various medical uses such as breathing oxygen.
The conventional high purity oxygen is composed of 99.5% oxygen, 0.5% argon, and essentially negligible nitrogen. However, that 99.5% oxygen purity includes trace amounts of heavy constituents present in the feed air such as krypton, xenon, and the hydrocarbons associated with the feed air. Since the cryogenic separation of feed air involves the separation by distillation, the separate components remain in the product streams dependent on their vapor pressure relative to one another. Of the primary components in the feed air, nitrogen is the most volatile, argon has intermediate volatility, and oxygen is the least volatile component. Additional trace components such as helium and hydrogen are more volatile than nitrogen and thereby exit the air separation plant with nitrogen rich streams. However, other trace components such as krypton and xenon are less volatile than oxygen and thereby will concentrate with the oxygen product. Similarly other heavy components such as propane, butane, and methane, are also less volatile than oxygen and will concentrate with the product oxygen. The trace components involved are generally in the parts per million purity range and do not normally constitute an impurity for conventional air separation processes.
Although the conventional high purity oxygen product is considered satisfactory for many industrial applications, it does not have sufficient purity specifications for some industrial applications. In particular, the electronics industry requires a higher grade product oxygen than the usual specification. The processes involved with this industry are such that trace amounts of heavy components such as argon, krypton, and the hydrocarbons will adversely impact on the quality of the final product. Accordingly, it is common for this industry to require oxygen product purity specifications that are considerably higher than the conventional high purity specification. Often the electronics industry applications require oxygen product with total impurity content of less than 100 ppm or even less than 50 ppm. Additionally, some heavy components such as krypton and hydrocarbons are especially detrimental to the quality of the products associated with the industry.
Furthermore, industrial applications such as the electronics industry often require elevated pressure nitrogen in addition to ultrahigh purity oxygen. The nitrogen is used as an inerting or blanketing gas and is needed at pressure for both flow distribution purposes and because some of the end use processes can operate at elevated pressure levels. The nitrogen is preferably produced at pressure directly from the air separation column, since any subsequent gas compression system has the potential to introduce undesirable particulates. The particulate content of the gases used within the electronics industry is important, since the particulates can settle and adversely affect the quality of the indicated electronic devices.
Although air separation processes are available to produce either ultrahigh purity oxygen or elevated pressure nitrogen products, there is a need to produce both products for the electronics industry. Such an air separation process would significantly improve the economics of the gas supply.
Therefore, it is an object of this invention to provide an improved process for cryogenic distillation separation of air.
It is a further object of this invention to provide an improved air separation process to produce ultrahigh purity oxygen.
It is a still further object of this invention to provide an improved air separation process to produce ultrahigh purity oxygen having a very low krypton content.
It is another object of this invention to provide an improved air separation process to produce ultrahigh purity oxygen having a very low hydrocarbon content.
It is yet another object of this invention to provide an improved air separation process to produce ultrahigh purity oxygen while also producing elevated pressure nitrogen.